The long answer: Here we go...the first and most common reason cats purr is because they're content. Cats purr when they're happy; some even purr while they eat. If your kitty is laying next to you purring up a storm, you can be pretty darn certain you've got a happy kitty on your hands.

The face of contentment.

Purring is not solely a response to contentment, however. Cats also purr when they're frightened, threatened, or giving birth! This seems to suggest that purring is also a way for a cat to comfort itself. Cats purr at about 25 Hz...the same frequency used for ultrasound therapy in humans. Since 25 Hz ultrasound therapy can help to manage pain in humans, it stands to reason that a cat's purr would do the same. It seems cats have their own built-in comforting and pain management system in the purr! This evidence suggests that a purring cat may have therapeutic benefits to humans as well (but of course we cat owners already knew that.)

It is also possible that cats under stress might purr as sort of a nervous gesture. For example, a cat being brought to the vet might purr even though it is clearly unhappy. It's almost as though the cat is saying, "See, I'm nice! Please don't hurt me." Just like people may smile uncontrollably or laugh inappropriately when we're nervous, some cats seem to purr to try and diffuse a bad situation.

There is still much controversy as to how and why the purr evolved. It doesn't seem we'll have a definitive answer anytime soon...so far, no cats are talking.